1. Field of the Invention
The present invention relates to a developing device that can be applied to an electro photographic type image forming apparatus typified by a copying machine or a printer. In addition, the present invention relates to an image forming apparatus equipped with the developing device.
2. Description of Related Art
A photoconductor drum is widely used as an image carrier in an electro photographic type image forming apparatus such as a copying machine or a printer. A typical image forming operation using a photoconductor drum is performed as follows. The surface of the photoconductor drum is electrostatically charged at a predetermined potential uniformly by an electrification device, and a light beam from an LED or the like of exposing device is projected to the surface of the photoconductor drum. As a result, the potential is attenuated optically in a part so that an electrostatic latent image of an original image is formed. This electrostatic latent image is developed by a developing device so that a toner image is formed. After the toner image is transferred to a sheet of paper, a cleaning device cleans the surface of the photoconductor drum so as to remove remaining toner. Then, a charge eliminating device emits charge eliminating light for eliminating electric charge as a preparation for the next image forming operation.
In the developing device that is a main part of the image forming operation as described above, a general method for developing an electrostatic latent image is to make toner adhere to a surface of a developing roller and to transfer the toner from the surface of the developing roller to the surface of the image carrier by electrostatic force. At this point, in order to realize a long life of the device and to maintain a good image quality, seal members are provided to both end portions in the axial direction of the developing roller so as to prevent the toner from leaking to the outside of the developing roller in the axial direction. An example of this developing device is disclosed in JP-A-2001-100525.
The method for preventing leakage of toner in the developing device as described in JP-A-2001-100525 is widely used because it can be easily adopted with a relatively simple structure. However, since this developing device has a structure in which the both end portions of the developing roller to which the seal members contact are also made of a conductive material, toner can adhere easily to these portions with electric action. In other words, toner can enter easily between the developing roller and the seal member. When toner enters between the developing roller and the seal member, the toner may be melted by frictional heat due to rotation of the developing roller and then may fix so as to affect badly and largely to life of the developing roller.
In order to solve this problem, there is proposed a method that makes it difficult for toner to enter between the developing roller and the seal member in the developing device. An example of the method is described in JP-A-2004-191430. According to the developing device described in JP-A-2004-191430, the end portions of the developing roller to which the seal members contact are made of a nonconductive material.
Since the end portions of the developing roller to which the seal members contact are made of a nonconductive material in the developing device described in JP-A-2004-191430, electric action does not work in these portions so that toner can hardly enter between the developing roller and the seal member. However, very complicated structure that requires much time and effort is necessary, which includes bonding a conductive material and a nonconductive material to each other on a bonding face having a special shape with adhesive and polishing the surface of the roller, for example, so that the end portions of the developing roller to which the seal members contact are made of nonconductive material. Therefore, cost of components and manufacturing cost are increased largely. It is desired to design the developing roller that can prevent toner from entering between the developing roller and the seal member, remaining, melting, and fixing there with a simple structure that can be realized at a low cost.
In addition, not only electric action but also surface roughness and surface hardness of the developing roller affect largely to transfer of toner. In other words, even if countermeasure is taken in consideration of electric action for preventing toner from entering between the developing roller and the seal member, the toner may be transferred by action of the surface roughness and surface hardness of the developing roller so as to enter between them. Furthermore, if the surface roughness of the developing roller is large, the toner that has entered between the developing roller and the seal member is difficult to go out and may remain. In addition, if the surface roughness is large, frictional heat as well as melting and fixing of the toner is easily generated between the developing roller and the seal member. The developing devices described in JP-A-2001-100525 and JP-A-2004-191430 do not have countermeasures against the generation of transfer of toner and frictional heat due to the surface roughness and surface hardness of the developing roller, so they tend to be in a state where toner can easily enter, remain, melt, and fix in a sealed portion of the developing roller. The melting and fixing of toner in the sealed portion of the developing roller may cause breakage of the developing roller or deterioration in image quality.